Work on a six-mile stretch of river around Rogers Island stopped late Friday after PCB levels several miles downriver exceeded federal drinking water safety standards for the second time in less than a week. PCBs are linked to cancer in animals.

After the stoppage, the levels dropped. On Tuesday, work resumed on three locations, one of which is inside a metal barrier called a sheet pile enclosure that is meant to confine stirred-up river sediments.

Three dredges returned to the task of scooping up PCB-tainted river bottom, down from the 11 working when chemical levels spiked above safety standards on Aug. 5. EPA blamed that rise on strong river currents caused by recent rains, coupled with unexpected debris on the river bottom that prevented dredge jaws from closing completely.

If subsequent testing this week shows PCB levels are acceptable, two additional dredging areas could be added as early as Thursday, Scopeck said. One additional dredge will be added each day if PCB levels remain below safety standards.

The unsafe PCB levels were found about 20 miles upriver from drinking water intakes used by the towns of Waterford and Halfmoon. Neither town has used the intakes since dredging began; both are using water purchased from Troy. Despite the PCB spikes upriver, chemical levels at the intakes have remained well within the safety standard.

Resuming work gradually will allow EPA and GE to watch how PCB levels change as more dredges are added. "We're learning a lot about the project as we go," Scopeck said. "This project is the only one of its kind."

The river cleanup is the largest project ever begun under the U.S. Superfund program. GE has assembled 11 dredges, 17 tugboats, 20 barges and more than 400 rail cars as well as skiffs, cranes and other machinery. As many as 90 vessels are on the river each day.

Since work began in mid-May, about 260 barges filled with PCB-tainted sediment removed from the river have been taken to a facility on the Champlain Canal, where the sediments are squeezed dry. The water is treated and pumped into the canal; the PCB sediments are shipped via rail cars to a toxic waste dump in Texas.

Critics of the dredging said the PCB levels show that EPA underestimated the work's consequences. "They are torturing this river," said Tim Havens, of Washington County Citizen Environmentalists Against Sludge Encapsulation.

But supporters of the project said EPA is reacting to conditions in a groundbreaking project that could drive PCB cleanups elsewhere in the world.

"I'm really impressed how EPA has handled the exceedences that have occurred," said Manna Jo Greene, environmental action director of the sloop Clearwater and Friends of a Clean Hudson. "They are resuming slowing and being cautious. This is exactly the right approach to take."